A concentrated photovoltaic cell which utilizes an inexpensive condensing lens or mirrors in order to condense the sunlight and converts the condensed sunlight into electric energy has been known. A band gap of a compound photovoltaic cell can be varied by controlling a composition ratio of compound semiconductor materials with relative ease. Therefore, studies are being conducted to improve energy conversion efficiency of the compound photovoltaic cell by absorbing the sunlight in a broader wavelength range.
Further, there is a photovoltaic cell formed by depositing a GaInP cell and a GaAs cell that are lattice-matched with GaAs onto a GaAs substrate and by depositing a GaInAs cell which has about 2% lattice mismatch with GaAs onto the GaInP cell and the GaAs cell via a lattice relaxation buffer layer, for the sake of improving the conversion efficiency (see, for example, Non-Patent Document 1).
Further, there is a photovoltaic cell formed by forming a GaInP top cell and a highly-doped p+-type layer onto a GaAs substrate, by forming a GaInAs bottom cell and a highly-doped n+-type layer onto an InP substrate, and by joining (bonding) the p+-type layer and the n+-type layer in order to form a tunnel junction layer. The GaInP top cell and the GaInAs bottom cell are bonded in series with each other via the tunnel junction layer (see, for example, Patent Document 1).
If resistance of a joint interface of the tunnel junction layer is high, it becomes harder for a tunnel current to flow through the tunnel junction layer. As a result, the conversion efficiency is decreased.
Since the resistance of the joint interface of the tunnel junction layer is not fully reduced, the photovoltaic cell disclosed in patent document 1 cannot achieve an adequate conversion efficiency.    Patent Document 1: US 2012/0138116    Non-patent Document 1: Proceedings of the 29th IEEE Photovoltaic Specialists Conference (2010) pp. 412-417.